Wound resistor



p 1970 R. G. MORRIS 3,529,273

WOUND RES ISTOR Origipal Filed July .31, 1967 INVENTOR! HUBERT EM 0221215.

United States Patent O Int. Cl. H01c 3/ 02 U.S. Cl. 338-302 6 Claims ABSTRACT OF THE DISCLOSURE A spark plug including a suppressor resistor mounted within its ceramic insulator to reduce undesirable radio frequency emanations. The internally mounted resistor is held within the axial bore of the insulator between a pair of resilient coil springs having a minimum effective inductance when compressed to secure the resistor in place. The spring is wound from a resilient conductor of rectangular cross section with the major dimension of the wire cross section parallel to the axis of the spring. Successive turns in a major portion of the spring are of reduced diameter so that these adjacent turns, when the spring is compressed, engage one another in electrically conductive contact to eifectively form a single turn or sleeve, thus reducing the effective inductance to a minimum, thereby reducing to a minimum any undesirable radio frequency emanations due to resonance of the inductance of the spring and capacitance inherent in the spark plug.

RELATED APPLICATION This is a division of application Ser. No. 657,235, filed July 31, 1967.

BACKGROUND OF THE INVENTION The use of internally mounted resistors in automotive ignition devices or spark plugs to suppress undesirable radio frequency emanations is well known in the art. While early constructions, such as that shown in U.S. Pat. 2,173,766, place the resistor within an ignition cable terminal, it has become common practice to include a re: sistor for suppression of such undesired radio frequency emanations within the ceramic insulator of the spark plug itself.

While carbon resistors are now commonly used with resistor type spark plugs, it is known that, at higher frequencies, wire-wound resistors have a higher efficiency in suppressing the undesirable emanations, due to the fact that carbon resistors decrease in impedance with increasing frequency. While the decreasing impedance characteristic of a carbon resistor may be overcome to some extent by using a very high D.C. resistance value, operational shortcomings due to the initial high resistance of a carbon resistor may be avoided by the use of a wirewound resistor which inherently has a higher impedance, and thus higher efiiciency in reducing unwanted high frequency radio wave emanations.

While wire-wound resistors have been used as the internal suppressor resistor in spark plugs in the prior art, as set forth, for instance, in U.S. Pat. 3,251,0l0, their adoption as internally mounted resistors has occasioned dificulty due to the fact that the physical length of the wire-wound resistor must be increased in order to accommodate end' caps or terminals which must be axially spaced apart a distance sufiicient to avoid flash-over at high voltages. This requires a spark plug manufacturer either to lengthen the ceramic insulator of the spark plug, or to make other design changes to accommodate the ice longer resistor. Secondly, wire-wound resistors must necessarily be secured with the ends of the wound resistor wire in electrical contact with the terminal and the spark plug center electrode.

Prior art constructions have Secured wire-wound Iesistors within spark plug insulators using a small coil spring at one end' of a capped resistor, which spring resiliently positions the resistor within the space available and also provides an electrical connection between that one end cap and the adjacent terminal or electrode. These prior art springs have been unsatisfactory for several reasons. Firstly, the spring at the end of the resistor, being an inductor, forms a resonant circuit with the capacitance inherent in the spark plug. When Shock excited by spark discharges, this circuit will oscillate and radiate strong radio-frequency signals. The values of capacitance and ind uctance are such that those undesirable signals will have a frequency falling within that portion of the frequency spectrum used for radio communication and other related services, thus causing undesired interference. Secondly, the prior art springs take up too much of the already diminished space available within the spark plug.

SUMMARY OF THE INVENTION It is an object of this invention to provide an improved spring terminal for use in securing spark plug resistors which is capable of performing its mechanical mounting functions within a minimum axial space and which also, when in its compressed state, has a minimum inductance so as to not be effective to create undesired interference, as previously explained.

The improved spring terminal of this invention is a wound coil spring having an upper and lower portion. The first or lower portion consists of a plurality of turns of equal diameter which are shorted together throughout their periphery, and with the effective inside diameter of this portion being substantially equal to the outside diameter of the wire-wound resistor so that, when the end of the wire-wound resistor is inserted within the first portion, the turns of the first portion are in electrical contact with the resistor wire and mechanically grip the end of the resistor. The second or upper portion of the improved spring terminal consists of a plurality of axially spaced turns with successive turns having a reduced diameter so that, when the second portion is compressed in an axial direction, each turn fits within its adjacent next larger turn such that all turns are in electrically conductive engagement with their adjacent turns throughout their periphery. When so compressed, the turns of the second portion take up a minimum axial space and, because all of the turns throughout the first and second portions are in electrically conductive engagement throughout their periphery, the effective inductance of the entire spring terminal is reduced to a minimum. In the preferred embodiment of this invention, the cross section of the conductive wire from which the spring terminal is wound is rectangular, with the major axis of the cross section being parallel to the axis of the spring terminal. As will be clearly seen from the detailed description below, the use of a conductor wire of this cross section further assres that the turns in the first section of the spring will grip the resistor in positive electrical engagement and that the turns in the second portion, when compressed, will engage adjacent turns throughout their periphery to reduce the effective inductance, as explained above.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a view in elevation, partly in cross section, of a complete spark plug showing a wire-wound resistor positioned within the axial bore of the ceramic insulator by means of improved spring terminals of this invention;

FIG. 2 is a view in elevation, on a greatly enlarged scale, of the improved spring terminal of this invention, shown in its non-compressed state;

FIG. 3 is a greatly enlarged portion of FIG. 1, showing a wire-wound suppressor resistor positioned between the terminal and the central electrode of the spark plug, with the spring terminals of this invention positioned on each end and in their compressed state', and

FIG. 4 is a view taken along line 4- 4 of FIG. 3 and showing the spring terminal of this invention in its compressed state.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring to FIG. 1, a spark plug of conventional design is shown, having a ceramic insulator with a wirewound resistor R positioned within its axial bore between a lower center electrode 11 and an upper terminal 12 by a pair of spring terminals '13. The insulator 10' is secured within the sprak plug shell body 14 which has a lower threaded portion 15 which engages the threads of a sprak plug hole in the combustion apparatus so that the firing electrodes 16 and 17 extend a proper distance within the combustion chamber.

Referring to FIG. 2, the improved spring terminal of this invention includes a first or lower portion 18 having a plurality of adjacent turns of the same diameter and a second or upper portion 19 having a plurality of turns with successive turns having a reduced diameter. In this preferred embodiment, with the spring terminal wound from a rectangular conductive wire with the major axis of the cross section of the wire parallel to the axis of the spring, the successively smaller turns of the second portion 19 are wound so that each turn has an outside diameter which is substantially the same as the inside diameter of the next adjacent larger turn. For example, in FIG. 2, the turn designated by reference numeral 20 has an outside diameter d which is substantially the same as the inside diameter of the next adjacent larger turn, designated by reference numeral 21. Thus, as the second portion 19 of the spring 13 is compressed in an axial direction, adjacent turns will fit within their next adjacent larger turn, as shown on the spring 13 illustrated in FIG. 3, so that the axial length of the second portion of the spring 13 may be compressed a substantial distance towards the first portion 18 of the spring 13.

This design allows the compressed axial length of the spring terminal of this invention to be a minimum. This is important, as previously explained, because wire-wound resistors must have an axial length greater than an equivalent carbon resistor, thus requiring the spark plug manufacturer to either reduce to a minimum size the mounting means for wire-wound resistors or to increase the length of the ceramic insulator. This latter expedient, for cost and other design reasons, is undesirable. Another advantage of the spring terminal of this invention is that, when the second portion 19 is compressed in an axial direction, to assume the configuration shown in FIGS. 3 and 4, each of the adjacent turns, 20 and 21, for instance, lie within the next adjacent larger turn with the entire periphery of the outer diameter of one turn (20) being in electrical contact with the entire periphery of the inner diameter of the next larger turn (21). Accordingly, the entire second portion 19 of the spring, when in compressed position, presents one complete turn or sleeve, thus reducing the inductance of the spring to an absolute minimum. By reducing the inductance of the spring to an absolute minimum, the frequency of the oscillations due to the resonance of the spring inductance and sprak plug capacitance is far above any frequency used in radio communication and related services.

Referring to FIG. 3, with the wire-wound resistor R held in place between the terminal 12 and the uppermost surface of the lower electrode 11 by means of a pair of the spring terminals 13 of this invention, the resistor R is cushioned against Shock or other vibrations and is in sound electrical engagement with both the terminal 12 and the electrode 11. As is apparent from FIG. 3, the spring terminal of this invention, while taking up a minimum of axial space, may still be compressed further, due to the unique design of the successively reduced diameter of the turns in the second portion 19. Thus, substantal dimensional variations in the axial distance available for the wire-wound resistor R and its spring terminals 13 may be accommodated by the use of spring terminals of this design. Additionally, because of the rectangular cross section of the wire of the preferred embodiment of this spring, it will be seen, from inspection of FIG. 3, that the engagement between the turns of the resistor R and the inside diameter of the turns making up the first portion 18 is over a relatively large area so that good electrical conductance is assured. Finally, because of the rectangular cross section of the wire used in the preferred embodiment, the engagement between the top or smallest turn of the spring terminals 13 and the adjacent surface of the terminal 12 or the electrode 11, at the positions designated by reference numerals 22 and 23 in FIG. 3, is over a relatively large area, thus assuring good electrical contact.

It will be apparent that the spring terminal of this invention may also be advantageously used to position resistors or other suppression devices in ignition cables, rotor or other components of an ignition system. Other advantages of this invention will be apparent to those skilled in the art and various modifications of the preferred embodiment described herein may be made without departing from the scope of the attached claims.

What I claim is:

1. A resistor comprising, in combination: a cylindrical core having opposed ends; a wire spirally wound about said core, said wire having two end portions and a predetermined electrical resstance between said end portions, said end portions of said wire being disposed adjacent opposite ones of said ends of said core; at least one spring wire terminal, said terminal comprising afirst portion having a plurality of turns, each turn with an inside diameter substantially equal to the diameter of said core, and a second portion having a plurality of axially spaced turns with successive turns having a reduced diameter whereby, when said second portion of said terminal is compressed in an axial direction, adjacent turns in said second portion are in electrically conductive engagement with each other to reduce the inductance of said terminal, said first portion of said terminal being attached coaxially over one end of said core in electrical contact with one of said end portions of said wire; and means for making electrical contact with the other end portion of said wire.

2. A resistor comprising, in combination: a cylindrical core having opposed ends; a wire spirally wound about said core, said wire having two end portions and a predetermined electrical resstance between said end portions of said wire being disposed adjacent opposite ones of said ends of said core; two spring wire terminals, said terminals each comprising a first portion having a plurality of turns, each turn with an inside diameter substantially equal to the diameter of said core, and a second portion having a plurality of axially spaced turns with successive turns having a reduced diameter whereby, when said second portion of said terminal is compressed in an axial direction, adjacent turns in said second portion are in electrical conductive engagement with each other to reduce the inductance of said terminal, said first portions of said terminals being attached coaxially over opposite ones of said ends of said core, said first portion of one of said terminals being in electrical contact with one of said end portions of said wire and said first portion of the other of said terminals being in electrical contact with the other of said end portions of said wire.

3. A resistor as claimed in claim 2, wherein said terminals are wound from wire having a generally rectangular cross section.

4. A resistor as claimed in claim 3, wherein in each terminal the major dimension of said wre cross section is arallel to the axis of said terminal.

5. A resistor as claimed in claim 3, wherein the opposed faces of adjacent turns normal to the axis of each terminal in said first portion of each terminal are in continuous conductve engagement with one another.

6. A resistor as claimed in claim 3, Wherein opposed faces of adjacent turns parallel to the axis of said terminals in said second portion of each of said termnals are in continuous conductve engagement with one another when said second portion is in an axially compressed condition.

References Cited UNITED STATES PATENTS 1,960,309 4/1934 Herrow 338-66 X ELLIOTT A. GOLDBERG, Primary Examiner U.S. Cl. X.R. 338-62, 66 

